1. Field of the Invention
The present invention is in the field of passive filtering of optical signals, e.g., in optical amplifiers, such as erbium-doped fiber amplifiers (EDFAs).
2. Description of the Related Art
Long period fiber gratings are described, for example, in A. M. Vengsarkar, et al., Long-Period Fiber Gratings as Band-Rejection Filters, Journal of Lightwave Technology Vol. 14, No. 1, January 1996, pp. 58-65. Such long-period fiber gratings are quickly becoming the filters of choice in several important communication applications. In particular, such filters are used to flatten the gain of EDFAs. The long-period fiber grating is based on the same principle and is fabricated with the same technology as standard reflection fiber gratings, which were invented nearly two decades ago. In a reflection fiber grating, a spatially modulated UV laser beam is launched onto one side of a single-mode fiber. Via complex mechanisms, now relatively well understood, the regions of the fiber exposed to a high UV intensity experience a refractive index change .DELTA.n in the vicinity of the core of the fiber, whereas the regions of the fiber receiving a low UV intensity experience a relatively small refractive index change. Thus, the fiber core index is periodically changed by .DELTA.n with a spatial period .LAMBDA. equal to the period of the spatially modulated UV beam.
To support a grating of the above-described type, the fiber must be sensitive to the UV, a property generally known as photosensitivity. The presence of sufficiently strong photosensitivity to form gratings generally requires the core to be doped with germanium oxide. The photosensitivity of fibers can also be enhanced, as described, for example, in P. J. Lemaire, et al., High pressure H.sub.2 loading as a technique for achieving ultrahigh UV photosensitivity and thermal sensitivity in GeO.sub.2 doped optical fibres, Electronics Letters, Vol. 29, No. 13, June 1993, pp. 1191-1193. Photosensitivity can be induced in non-Ge-doped fibers by loading the fiber with hydrogen prior to UV exposure.
In photosensitive reflection fiber gratings, the period .LAMBDA. is selected such that the signal traveling in the core (the fundamental mode LP.sub.01) is coupled to the LP.sub.01 mode traveling in the other direction. Thus, the forward-traveling signal is coupled (partially or fully, depending on the grating) to the backward-traveling signal. The grating acts as a reflector. For this reflection to take place, .LAMBDA. must be fairly small. For example, A should be on the order of a fraction of the signal wavelength.
In the second type of photosensitive fiber gratings, the long-period grating, the forward LP.sub.01 signal mode is not coupled to the backward mode. Instead, the forward LP.sub.01 signal mode is coupled to a cladding mode. Since the cladding modes are strongly attenuated by the jacket surrounding the fiber, the LP.sub.01 mode is attenuated. However, if a broadband signal travels in the fiber, for a given period .LAMBDA., only certain wavelengths will be coupled to cladding modes. For other wavelengths, there may be no cladding modes to which the signals at those wavelengths can be coupled. Thus, the signals at those wavelengths experience no loss. Therefore, the grating attenuates only some wavelengths (i.e., the grating acts as a wavelength-dependent filter). To couple the LP.sub.01 mode to a cladding mode, the period .LAMBDA. for a long-period grating must be much longer than .LAMBDA. for a reflective grating. Typically, .LAMBDA. for a long-period grating is on the order of tens to several hundred microns. The transmission versus wavelength of such long-period grating filters exhibits multiple transmission notches or dips. By appropriately selecting .DELTA.n, .LAMBDA., and the grating length, the depth and (nearly periodic) location of the various dips in the filter function can be advantageously controlled.
Long-period gratings are especially important to flatten the gain of an EDFA. A standard scheme is to place a filter with a suitable filter function at some optimum location near the EDFA. It turns out that for EDFAs, the desired filter functions exhibit the kind of shape and amplitude that can easily be generated with a long-period fiber grating.
One difficulty with these exemplary long-period fiber gratings is that an expensive UV laser is required to fabricate the gratings. For example, an excimer laser having a wavelength around 248 nanometers and costing approximately $60,000 has been utilized to fabricate such gratings. The fabrication of fiber gratings is consequently reserved to the relatively few laboratories having such equipment. A second difficulty is that the response of a fiber to a particular UV exposure depends strongly on the chemistry, the composition and the thermal history of the fiber in ways that are too complex to model and that must be determined empirically. Thus, the fabrication process is not easily reproducible from one fiber to the next. When presented with the task of fabricating a particular long-period fiber grating filter in a particular fiber, manufacturers must go through several iterations, often based on trial and error, which adds cost and delays to the manufacturing of such filters. A third difficulty is that reproducibility requires particularly careful control of the UV laser power and fiber parameters, which can be difficult.
There are other ways of constructing long period fiber gratings. One is to arc (with an electrical arc, e.g., from a commercial fiber splicer) across the fiber, which locally releases the stress built into the fiber and locally modifies the index. The process is repeated at regular intervals, with a period .LAMBDA., along a length L of fiber. Another method is to arc and pull the fiber slightly, which deforms it a little and thus perturbs the mode index. These two methods work for any fiber without prior processing (unlike photosensitive gratings) but they are also permanent. However, a main practical difficulty that arises in the manufacturing of long period fiber gratings is that the transmission function of the fiber is difficult to control since it depends on the difference between two large numbers of comparable magnitude, namely n.sub.eff and n.sub.cl.sup.m, which are the effective index of the LP.sub.01 core mode and the effective index of the m.sup.th cladding mode, respectively. (See Equation (1) below.) Both n.sub.eff and n.sub.cl.sup.m depend strongly on fiber parameters, and small variations in either or both of these indices that would have little effect in a reflection grating have a large effect on the long period fiber grating's wavelengths of peak attenuation, making it difficult to reproducibly manufacture gratings of a desired transmission.
Another kind of fiber device relevant to the present invention is a periodically stressed device, like the ones invented at Stanford University in the 1980s. (See, for example, R. C. Youngquist, et al., Two-mode fiber modal coupler, Optics Letters, Vol. 9, No. 5, May 1984, pp. 177-179; and R. C. Youngquist, et al., Birefringent-fiber polarization coupler, Optics Letters, Vol. 8, No. 12, December 1993, pp. 656-658.) When pressure is applied to a glass fiber, its core is deformed. Also, the index of the fiber changes via the photoelastic effect. The first effect has been used in a two-mode fiber to produce a mode coupler. (See Youngquist, et al., Two-mode fiber modal coupler, cited above.) A two-mode fiber is a fiber in which light can propagate in one or both of two transverse modes (e.g., the LP.sub.01 mode and the second-order mode LP.sub.11). Pressure is applied periodically along the fiber by pressing a mechanical plate against the fiber. The metal plate is made of a periodic comb of square ridges. Each ridge locally squeezes and deforms the fiber. At the beginning and at the end of each stressed region, the deformation couples a little power from the LP.sub.01 to the LP.sub.11 mode. For this effect to accumulate and to cause the transfer of substantial power between modes, the phase between coupling points must be correct. In particular, the ridge period must be substantially equal to the beat length between the two modes. For example, the device demonstrated in Youngquist, et al., Two-mode fiber modal coupler, cited above, had a proper period of approximately 430 microns and produced up to 40 dB coupling from the LP.sub.01 mode to the LP.sub.11 mode.
The second effect is index change with pressure. The second effect has been used to produce a polarization coupler using a high-birefringence fiber. (A high birefringence fiber is a fiber in which light can propagate in one or both of two well-defined linear polarizations.) The polarization coupler couples substantially all the power from one polarization to the other. (See, for example, Youngquist, et al., Birefringent-fiber polarization coupler, cited above.) For such strong coupling to take place, a mechanical plate made of a periodic comb of square ridges is pressed against o the fiber. Preferably, the comb is placed at 45.degree. to the fiber birefringence axes. Each ridge causes an asymmetric perturbation in the fiber index, which perturbs the birefringence of the fiber in the portion affected by the comb. The index perturbation allows a small amount of coupling to take place at the beginning and at the end of each stress region. Full coupling occurs when the period of the stress region is equal to the beat length between the two polarization modes. The described device has a ridge period of approximately 820 microns and yields up to 25 dB coupling of one polarization to the other. (See, Youngquist, et al., Birefringent-fiber polarization coupler, cited above.)
Tachibana et al. constructed optical notch filters having lengths of 220 mm and 390 mm by sandwiching a segment of amplifier fiber between a mechanical grating and a flat plate. (See, Tachibana, et al., Gain-shaped erbium-doped fibre amplifier (EDFA) with broad spectral bandwidth, Proceedings of Conference on Optical Amplifiers and their Applications, Optical Society of America Trends in Optics and Photonics Series, MDI, pp. 44-47, Aug. 6, 1990; Tachibana, et al., Erbium-doped fiber amplifier with flattened gain spectrum, IEEE Photonics Technology Letters, vol. 3, pp. 118-120, February 1991.) However, these lengths may be unsuitable for certain applications. Furthermore, the configuration suggested in these references is polarization dependent, which is unsuitable for amplifier applications.